Why is one eyelid drooping?

Age-related levator dehiscence

The upper eyelid is lifted by a muscle called the levator palpebrae superioris. This muscle tapers into a broad, flat tendon that attaches to the front of the cartilage plate inside the eyelid, called the tarsus. As people age, this tendon can gradually stretch or partially disinsertion from its attachment, a process called levator dehiscence. When this happens, the eyelid drops because the mechanical connection between the muscle and the eyelid has become lax.

Levator dehiscence is the most common cause of ptosis in adults. It often affects one side before the other, or affects both sides to different degrees, which is why the drooping can appear one-sided. The condition develops slowly over years and is associated with ageing, long-term contact lens wear, and previous eye surgery. It is entirely benign in itself, though depending on how much the eyelid drops, it can obstruct the upper visual field or create a tired, asymmetric appearance.

The levator function, which is measured by how far the eyelid moves from full downgaze to full upgaze, is preserved or only mildly reduced in levator dehiscence, because the muscle itself is intact. This distinguishes it from conditions where the muscle is genuinely weak.

Ptosis versus excess eyelid skin

These two conditions are frequently confused, including by patients who have been told they need blepharoplasty when what they actually have is ptosis, or vice versa. The distinction matters because the treatments are different, and doing the wrong one will not solve the problem.

Ptosis is a mechanical problem with the muscle or tendon that lifts the eyelid. The eyelid margin itself is lower than normal. Excess eyelid skin (dermatochalasis) is a separate issue where the skin above the eyelid loses its elasticity and folds forward, sometimes drooping far enough to hang over the eyelid margin and obstruct vision. The eyelid margin in dermatochalasis is in the normal position; it is the overlying skin that is heavy and redundant.

In practice the two commonly coexist in older patients, which is why careful examination is needed to determine the relative contribution of each to the problem. Blepharoplasty removes the excess skin. Ptosis surgery repositions the eyelid margin by reattaching or shortening the levator tendon. If both are present and only the skin is addressed, the patient is left with a better skin contour but the same low eyelid margin.

Myasthenia gravis

Myasthenia gravis is an autoimmune condition in which the body produces antibodies that block the receptors at the junction between motor nerve fibres and muscle cells. This disrupts the transmission of nerve signals to the muscle, causing weakness that is characteristically variable and fatigable: it worsens with sustained use and improves with rest.

The eyelid and eye movement muscles are among the most commonly affected, and ptosis is one of the most frequent presentations of myasthenia. The drooping often varies during the day, typically worse in the evening after the muscles have been used, and better in the morning after rest. It may also shift from one side to the other, or be bilateral but asymmetric. If you notice that your droopy eyelid is noticeably better when you have just woken up and progressively worse as the day goes on, myasthenia is worth considering.

Double vision alongside variable ptosis is another characteristic feature. Myasthenia is diagnosed with a blood test for acetylcholine receptor antibodies, along with nerve stimulation studies. It is a treatable condition, and most patients achieve good control of their symptoms with appropriate medication.

Horner syndrome

Horner syndrome causes a distinctive cluster of signs: a smaller pupil on the affected side, a slightly droopy upper eyelid, and sometimes a slight elevation of the lower lid. The drooping in Horner syndrome is typically mild, usually only one to two millimetres, which distinguishes it from more significant ptosis caused by levator problems.

The underlying cause is disruption to the sympathetic nerve supply to the eye, which runs a long route from the brainstem down through the neck and chest. A new Horner syndrome, particularly if accompanied by pain in the neck or face, warrants investigation to identify the cause, as it can rarely indicate a problem with the carotid artery or a lesion in the chest or brain.

Third nerve palsy: the emergency to rule out

The oculomotor nerve (third cranial nerve) supplies four of the six muscles that move the eye, the main eyelid elevator (levator palpebrae superioris), and the muscle that constricts the pupil. A complete third nerve palsy therefore produces a markedly droopy eyelid, a large dilated pupil, an eye that deviates outward and downward, and double vision.

The combination of a new droopy eyelid with a large, unreactive pupil on the same side is an emergency, because it can indicate an aneurysm compressing the third nerve at the base of the brain. The pupil-constricting fibres run on the outer surface of the nerve and are typically compressed first when there is external pressure from an aneurysm. A pupil-involving third nerve palsy must be assessed urgently by a neurologist or in a neurosurgical unit the same day.

Treatment for ptosis

The treatment for ptosis depends on its cause and severity. Age-related levator dehiscence, the most common variety, is treated surgically by reattaching or advancing the levator tendon to its normal position on the tarsus. This is a precise and highly effective procedure, typically carried out under local anaesthetic as a day case. The result is a raised, more symmetrical eyelid margin.

Myasthenia is managed medically, with medications that improve the transmission of nerve signals to the muscle, including pyridostigmine and, in some cases, immunosuppressive therapy. Surgery is generally avoided in active myasthenia because the results are unpredictable when the underlying muscle function is variable.

If you have a droopy eyelid that is causing visual obstruction, creating a tired or asymmetric appearance that is bothering you, or that has appeared suddenly, a consultation with an oculoplastic surgeon is the appropriate starting point.